Prior to use, any radiation detector used for intensity measurements, such as a spectrometer, must be calibrated. Radiometric calibration presents special problems for X-ray analysis, particularly for soft and ultrasoft X-rays having wavelengths in the range of 2 to 200 angstroms, because of the problems inherent in focussing the radiation and because of the nature of the apparatus used for generating the radiation. The radiometric calibration of such instruments is, as a result, time-consuming and expensive, even to achieve an accuracy of ten or twenty percent. Because of the sensitive nature of instrumentation in this region, the entire result can be rendered useless quickly by a simple vacuum accident or over a period of weeks by contamination. In particular, the X-ray range of the spectrum between 10 angstroms and 120 angstroms, or about 100 EV to 1,000 EV is beyond the truly useful range of the available standard sources, such as synchrontron or electron storage ring and standard detectors, such as the NBS diodes. In addition, there is a problem of compatibility of such instruments with a host calibration facility. Finally, because the calibration facility is normally remote from the instrument to be calibrated, it is impossible to spot-check calibrations when desired.
Existing methods of calibration include sending the detector to be calibrated to the National Bureau of Standards, but the National Bureau of Standards will only calibrate instruments meeting their high structural standards, and the difficulties involved in shipping and transporting such an instrument are obvious. Other known methods of calibration include using a reference detector, such as an absolute detector. In this method, a single beam source is used, and the absolute detector is placed between the detector to be calibrated and the source. Once appropriate readings have been taken, the absolute detector is removed physically and the other detector may be calibrated. This procedure is cumbersome, and does not always produce accurate results because of the time delay between the two measurements taken. In addition, the necessity of physically moving the absolute detector increases the likelihood of error. A somewhat similar system is described by Morgan (Morgan, F. J., A. H. Gabriel and M. J. Barton, 1 J. Phys. (E) 998 (1968)) in which the absolute detector is placed between the detector to be calibrated and the source, but is offset at a slight angle of about 5-10 degrees below the detector to be calibrated. The radiation flux received by the absolute detector is thus not exactly equal to that received by the detector to be calibrated, and thus a certain amount of error is inherent. The accuracy of the reading is proportional to the distance of the two detectors from the source. Obviously, a large, cumbersome and expensive apparatus is required for a high degree of accuracy, and because of the necessarily long path for the radiation, a high power X-ray source is required.
In addition to the system developed by Morgan, most X-ray calibration devices utilize a high power X-ray source. Examples are the well known Henke source or variations thereof as shown by Morgan at page 999 or an electron synchrotron as used by Tomboulian. (Tomboulian, D. H., and P. C. Hartman, 102 Phys. Rev. 1423 (1956)). All of these high power sources require water cooling to prevent melting of the anode and other parts of the apparatus. Also, because the source areas of most of these known high power sources are large and poorly defined, two apertures are required to produce a well defined X-ray beam, one close to the source to limit the beam, and another aperture spaced from the source to limit the angle of the beam. It is nearly impossible to generate two beams of the same intensity at different angles with such a source.
Each of the above described methods of calibration is time-consuming and requires expensive and complex apparatus, and thus few laboratories can be equipped to provide the accuracy of radiometric calibration required for most experiments and analyses.